For protecting our living environment, sewage, being the wastewater released by residences, businesses and industries in a community, must be treated for remove as much of the suspended solids and pollutants as possible before the remaining water, called effluent, is discharged back to the environment. Sewage treatment is a multi-stage process that can include a primary physical/chemical procedure, a secondary biological procedure, and a tertiary fine filtration/adsorption/concentration procedure. Recently, the use of algae in this multi-stage wastewater treatment process is very common and is performed in a way similar to industrial algae cultivation for separating a solid waste (or treated sludge) for disposal from an environmentally safe fluid effluent, and the same time producing algae biomasses to be harvested. Moreover, the harvest of algae biomasses consists of separating algae from the growing medium, drying, and processing it to obtain the desired product. Generally, not to mention that the procedure of algae harvesting and sludge separation is considered to be the key process in the wastewater treatment process, the equipments and facilities required this procedure of algae harvesting and sludge separation can be the most expensive comparing to those other apparatuses used in the wastewater treatment process.
The belt filter press is an industrial machine, which is commonly used for solid/liquid separation processes, particularly the dewatering of sludges in water treatment. The process of filtration is primarily obtained by passing a pair of filtering cloths and belts through a system of rollers. Operationally, the feed sludge to be dewatered is introduced from a hopper between two filter cloths (supported by perforated belts) which pass through a convoluted arrangement of rollers. As the belts are fed through the rollers, water is squeezed out of the sludge. When the belts pass through the final pair of rollers in the process, the filter cloths are separated and the filter cake is scraped off into a suitable container. Generally, after each press, the filter cloths are cleaned by means of water sprays positioned on the return section of the belt for preventing sludge buildup on belt and filter cloths. Nevertheless, not only the cleaning of the filter cloths by water spray is an operation using a lot of clean water and energy, but also it is not guarantee to clean the filter cloths completely and thus in most case, the dewatering performance of those used filter cloths is decreasing with times and eventually required to be replaced. In addition, the shortcomings of a conventional belt filter press further includes: only 40% of the perforated belt is working for dewatering at any given time while allowing the other 60% to be left idle, but only moving along with the rotating rollers; and during a dewatering process in a conventional belt filter press, the sludge to be dewatered is being brought along to move with the moving belt, resulting that the conventional belt filter press will waste most of its energy consumption just to move the heavy load of the sludge; the lifespan of the filter cloths is shorter compared with other device using cloth media since the filter cloths are subjected to a plurality of pressing steps while being twisted, turned and pulled in large angles during a dewatering process; belt filter presses will require to work with the cooperation of a vacuum filtering system for minimize offgas and effluent during operations, and if there is no such system, the belt filter presses can only perform well at lower speed; and a conventional belt filter presses is only suitable for dewatering sludge with comparatively larger solid particles, but is not suitable for algae cultivation industry for harvesting and concentrating delicate algae.
In the conventional algae cultivation, algae generally are harvested by means of mechanical centrifugation. Accordingly, a centrifuge that is driven by a motor is used for removing excess water from a batch of algae harvest by rotating at a high speed. That is, the high speed of rotation creates a high centrifugal force for the water in the algae batch which causes it to be pulled to the outside of the spinning portion of the centrifuge and away from the algae. Thereby, no filter cloth is used and required for cleaning. However, in addition to the high initial cost and operation cost, such centrifuge is also not favored to be used in the algae harvesting since it can be very noisy during operation. Other than that, there is a thin film separation process being developed for algae harvesting and concentration. However, such thin film separation process still has the following disadvantages: small process capacity, high equipment cost, and the thin film can easily be clogged and thus required to be either replaced or processed by an expensive reverse rinse procedure. Consequently, the thin film separation process is not popular.
There are already many studies focused on the improvement of dewatering devices. Nevertheless, none of those studies can provide a new dewatering method different from the conventional belt filter press. That is, the problems of high energy consumption and wear-and-tear in filter belt still remain in the aforesaid studies since the sludge to be dewatered is still being brought along to move with the moving belt during a dewatering process, and thus most of its energy consumption will be wasted just to move the heavy load of the sludge, not to mention that the heavy load of the sludge will cause the supporting belt to withstand a larger pulling force from a convoluted arrangement of rollers. In addition to the lack of means for solving the problems of high energy consumption and wear-and-tear in filter belt, most of the dewatering apparatuses that are currently available require their filter belts to be made of materials with high mechanical strength, and consequently, not only the selection of materials for filter belt is limited, but also the manufacturing cost is increased.